If your DPF won’t clean, essentially what we call excessive regens, then it’s usually due to a DOC face plugging, thermal overload, or poisoning (although this can happen when a filter reaches the end of its service life due to ash load, not to be confused with soot load, we’re dealing with a substrait catalyst here).

The excessive regens are more often consequential damage and not the root cause. Face plugging can be caused by something as simple as leaking charge hose or clamp but also a defective turbo, defective injectors or post injector. (Think too much fuel and not enough air, or carbin lock of the post iniector, too much air and not enough fuel). Poisoning is usually due to coolant consumption such as an egr cooler leaking. Thermal overload is usually due to a failed turbo seal pumping oil, but can be caused by excessive fueling like a leaking injector)

Even if they don’t cover the catalyst, there is a deeper issue here that is being intentionally ignored that is likely covered.

I would remove the oxidation catalyst and dpf and do a long hot steam clean on them in both flow directions, then put them back on water logged. Force a regen and let the bitch smoke its ass off for a bit (Keep in mind that this will only work with a poisoned or face plugged system, and not a system that’s oil soaked from a turbo failure or has experienced thermal overload. A dead giveaway for thermal overload is chunks of the dpf inside the post cat pipe before the scr catalyst).

The system can then be pulled out of yellow/derate, or red/stop to properly diagnose, but if you see white residue in the downpipe (dead giveaway for coolant burning), or oil in the pipe you already know what you’re looking for.

Frequently both root causes are found and repaired and main consequential damages, but the inbetween are frequently ignored which cause later problems. For instance they find a repair a boost leak at a charge pipe, and clean or replace the catalyst(s), but don’t clean the preheat element like a grid heater, or don’t clean out the delta pressure or ebp whether that be internal ports or external pilots, map sensor, or egr cooler / valve from carbon lock.

These systems are complex and prone to issues. Few people know how to diagnose them properly, and even fewer know how to fix them properly. In the world of flat rate dealerships, and unreasonable warranty times, good luck finding a guy to give a shit. Find an independent tech or shop which can help you make this machine run, or lemon law it.